half the impurities... and almost double the price. Still unexpensive though. Worth to go for the ACS.
For i.e. I have the 2014 catalogue here of one of the biggest german chemical manufacturers (they don't sell directly to the public). List prices for KI :
250g ACS (99,5% pure) : 70,45 EUR
250g USP (99,0% pure) : 50,45 EUR
And then they have their own purified version, extra extra extra purified : 99,999% (5N)
25g 84,85 EUR. THAT is expensive.